Avalanche transistor circuits offer several features which are desirable in fast counting systems; simplicity, versatility, fast response, and large charge gain. This paper describes the evolution of avalanche circuits which retain the desirable features but which do not suffer from the two defects of the basic avalanche circuit: low duty cycle, and a pulse propagation delay which is dependent upon input. Several transistor types which are suitable for fast avalanche circuits are compared and comments are made about the application of these transistors. Described also are several useful circuits which may be constructed with avalanche transistor techniques.